A conventional hybrid vehicle has, as a drive power generator for generating drive power to drive the drive shaft of the vehicle, an internal combustion engine, a rotary electric machine and an electric charge storage device, which transfers (supplies and receives) electric power to and from the rotary electric machine. In this hybrid vehicle, energy required for travel is charged to the charge storage device at charge stations, at which an electric power supply (commercial electric power source or the like) external to the vehicle is available (for example, patent documents 1 and 2). By using the electric power supply, which is external to the vehicle, fuel cost can be reduced and the amount of exhaust emission during travel can also be reduced.    (Patent document 1) JP 8-154307A    (Patent document 2) JP 2007-99223A
In the hybrid vehicle according to patent document 1, the output of the rotary electric machine is limited and the internal combustion engine is used to travel, when a predetermined period elapses after charging by the an external charging device. When the output of the electric rotary machine is limited, an alarm is issued to a driver, so that the driver is urged to perform charging by the external charging device. In this hybrid vehicle, only a travel time period after the charging by the external charging device is used as an index for urging the charging by the external charging device. The charging is urged in some instances, even when the charge storage device has a high state of charge (SOC) and cannot store so much electric charge any more. Since the electric power cannot be charged from the external power source, the amount of exhaust emission cannot be reduced sufficiently.
In the hybrid vehicle according to patent document 2, a travel distance to a next external charge point is acquired from a car navigation device and the upper and lower control limit values of the SOC of the charge storage device are set lower as the vehicle approaches the external charge point. With this setting, the SOC can be lowered at the time of arriving at the external charge point, and the charge storage device can be charged with more electric power at the external charge point. In this hybrid vehicle, the upper and lower control limit values of the SOC of the charge storage device are lowered as the hybrid vehicle approaches the next external charge point. The device is charged and discharged, when the SOC reaches the lower and upper control limit values, respectively.
In some cases, the hybrid vehicle has to stop for a long time in traveling toward the charge point or travel at low speeds by using the electric motor. As a result, the SOC tends to fall below the lower control limit value before the hybrid vehicle arrives at the next external charge point. For this reason, in the hybrid vehicle according to patent document 2, the motor/generator is driven by the drive power of the engine to generate electric power-even when the actual SOC does not fall to a target control limit value of the SOC, which is set for the next external charge point. The charge storage device is charged with the electric power thus generated so that the actual SOC is maintained higher than the lower control limit value at each time. As a result, the period of operating the engine cannot be shortened sufficiently. In addition, the engine must be operated at operation points, at which fuel consumption efficiency is low, during travel stop (idling), low speed travel and the like of the vehicle. Thus, the fuel cost and exhaust emission cannot be reduced sufficiently.